Digital broadcasting systems, such as the digital television system, have been under development, and they are seen to gradually replace the analog broadcasting systems. This is, among other things, because of their ability to provide new types of services and better quality of service compared to the analog broadcasting systems.
One of the digital broadcasting systems currently being under standardisation by the European Telecommunications Standards Institute (ETSI) is the Digital Video Broadcasting (DVB) system. In the DVB system digital video is broadcasted using satellite, cable and/or terrestrial infrastructures.
The DVB system using terrestrial infrastructure is defined as DVB-T (DVB-Terrestrial) system. In DVB-T transmission digital data is modulated into a radio frequency (RF) signal. The modulation method used is the COFDM (Coded Orthogonal Frequency Division Multiplex) modulation. The modulated DVB-T signal is transmitted from a DVB-T transmitter. The transmission occurs over a DVB-T radio channel. The transmitted DVB-T signal is received at a so called set top box (STB) containing a DVB-T receiver. The DVB-T receiver demodulates the received DVB-T signal in order to regenerate the digital data. The digital data may contain for example an MPEG-2 (Moving Picture Experts Group) coded video stream.
FIG. 1 shows funtional blocks of a DVB-T receiver according to prior art. The DVB-T receiver 100 shown in FIG. 1 functions according to a well known superheterodyne principle.
A COFDM modulated radio frequency DVB-T signal sent from a transmitter and received via an antenna is conveyed to a low noise amplifier 101 of the DVB-T receiver 100. In the low noise amplifier 101 the DVB-T signal is amplified in order to raise the power level of the received DVB-T signal. The amplified DVB-T signal is conveyed to a tracking filter 102. The tracking filter 102, which is an adjustable band pass filter, filters the DVB-T signal to attenuate the frequency components that lie outside the frequency band which is desired to be received. From the tracking filter 102 the DVB-T signal is conveyed to an adjustable RF amplifier 103 which amplifies the filtered DVB-T signal in order to compensate losses caused in the tracking filter 102. From the adjustable RF amplifier 103 the DVB-T signal is conveyed to a second adjustable tracking filter 104 for band pass filtering. The purpose of the second adjustable tracking filter 104 is to attenuate the image frequencies of the DVB-T signal. The filtered DVB-T signal is conveyed from the second tracking filter 104 to a first input of a down conversion mixer 105.
A local oscillator 111 generates a local oscillator signal. A phase locked loop 112 controls the frequency of the local oscillator signal as well as the pass band of the previously mentioned adjustable tracking filters 102 and 104. The frequency of the local oscillator signal is controlled with the aid of a feedback from the local oscillator 111 to the phase locked loop 112 and with the aid of a reference oscillator signal which a reference oscillator 113 provides for the phase locked loop 112. The local oscillator signal is conveyed to a second input of the down conversion mixer 105.
The down conversion mixer 105 mixes the DVB-T signal coming from the second tracking filter 104 with the local oscillator signal in order to convert the DVB-T signal down into the frequency band of an intermediate frequency (IF). The down converted DVB-T signal is conveyed to an IF amplifier 106 which amplifies the down converted DVB-T signal. From the IF amplifier 106 the DVB signal is conveyed to an IF filter 107 which band pass filters the amplified DVB-T signal. The IF filter 107 is a band pass filter of a fixed pass band the width of which is substantially the same as the width of one DVB-T channel. The pass band of the IF filter 107 has steep edges so as to strongly attenuate the frequency components that lie outside the width of the DVB-T channel. From the IF filter 107 the DVB-T signal is conveyed to an adjustable IF amplifier 108 which amplifies the signal before the signal is conveyed to a COFDM demodulator 109. The COFDM demodulator 109 is a digital demodulator block which demodulates the received COFDM modulated DVB-T signal so as to form the (original) digital data. From the COFDM demodulator 109 the digital data may be conveyed, for example, to an MPEG-2 decoder or to another appropriate functional block.
The COFDM demodulator 109 itself may comprise a plurality of functional blocks (not shown) the operation of which is generally known to a person skilled in the art. Typically, the COFDM demodulator 109 contains blocks for performing analog-to-digital conversion (ADC), automatic gain control (AGC), Fast Fourier Transform (FFT), channel estimation and correction, and channel decoding.
Of the mentioned blocks the ADC is used to convert the received DVB-T signal from an analog form to a digital form. The AGC block controls with a feedback control signal AGC-1 the gain of the adjustable RF amplifier 103 and with a feedback control signal AGC-2 the gain of the adjustable IF amplifier 108 so as to optimise the received DVB-T signal voltage level so that the received DVB-T signal fits to an ADC conversion window. The gain is adjusted so that the ADC does not clip, that is the signal voltage of the DVB-T signal does not exceed an upper limit (nor go below a lower limit) of the ADC conversion window, because information carried by a clipped portion of the DVB-T signal will be corrupted or completely lost.
The FFT block is used to transform the analog-to-digital converted signal from time domain to frequency domain. The channel estimation and correction block is used to determine a transfer function H(f) of the DVB-T channel and, based on the transfer function H(f), to correct the effects that the transmission path causes to the DVB-T signal. The channel correction is typically performed by multiplicating the DVB-T signal with a function 1/H(f) which is an inverse function to the determined transfer function H(f). The transfer function H(f) is determined based on particular pilot signals. The pilot signals are signals which are transmitted together with the DVB-T signal but whose transmission amplitudes and location in the spectrum are known, in advance, to the DVB receiver.
The channel decoding block is used to reverse coding and interleaving operations performed at the DVB-T transmitter. For example, error correction (relating to errors occurred in the transmission path) is performed in the channel decoding block.
With terrestrial digital video broadcasting it is possible to achieve a good quality data transfer even if the DVB-T receiver is mobile. However, problems may occur with the mobile DVB-T receiver due to the small size requirement of the mobile communication devices and due to the desire to use internal integrated antennas and rapid digital electronics such as microprosessors/controllers in these devices. Namely, the digital electronics produce radio frequency interference which tend to couple to the device's own antenna(s) thus degrading the performance of the device in the form of bigger overall bit error rates and in the form of reduced receiver sensitivity in at least some of the receiving channels.
One commonly used solution for reducing the effects of this kind of self-created digital interference has been to shield the components causing radio frequency digital interference for example with metal cups.
Another possibility for reducing the effects of self-created digital interference is to make the device bigger in order to have a better separation between the receiving antenna and the sources of the digital interference.
Another possibility is to use external antennas instead of internal antennas. However, it is in many cases desirable to use internal antennas.
The self-created digital interference is considered to be rather stable in frequency domain. The digital interference typically consists mainly of clock oscillator signals, the harmonic signals of the clock oscillator frequency and/or the intermodulation products of the clock signals. The spectrum of the digital interference is considered to be a stable comb-like line spectrum rather than a continuous spectrum. In narrowband systems it may thus be possible to avoid the effects of the digital interference already in the designing phase of the device by designing the device so that no digital interference signals are present in the used (frequency) band. However, in broadband systems (such as the DVB-T system in which the total used bandwidth is around 400 MHz, more specifically 474 MHz–858 MHz) this kind of designing procedure is not very useful due to the broadness of the used bandwidth.